Recent advances in genome sequencing technologies and analysis have yielded significant insights into the genetic underpinning of biological activities in many diverse areas of nature, ranging from prokaryotic biosynthetic pathways to human pathologies. To fully understand and evaluate the vast quantity of information produced by genetic sequencing technologies, equivalent increases in the scale, efficacy, and ease of technologies for genome and epigenome manipulation are needed. These novel genome and epigenome engineering technologies will accelerate the development of novel applications in numerous areas, including biotechnology, agriculture, and human therapeutics.
Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and the CRISPR-associated (Cas) genes, collectively known as the CRISPR-Cas or CRISPR/Cas systems, are currently understood to provide immunity to bacteria and archaea against phage infection. The CRISPR-Cas systems of prokaryotic adaptive immunity are an extremely diverse group of proteins effectors, non-coding elements, as well as loci architectures, some examples of which have been engineered and adapted to produce important biotechnologies.
The components of the systems involved in host defense include one or more effector proteins capable of modifying DNA or RNA and a RNA guide element that is responsible to target these protein activities to a specific sequence on the phage DNA or RNA. The RNA guide is composed of a CRISPR RNA (crRNA) and may require an additional trans-activating RNA (tracrRNA) to enable targeted nucleic acid manipulation by the effector protein(s). The crRNA consists of a direct repeat (DR) responsible for protein binding to the crRNA and a spacer sequence, which may be engineered to be complementary to a desired nucleic acid target sequence. In this way, CRISPR systems can be programmed to target DNA or RNA targets by modifying the spacer sequence of the crRNA.
CRISPR-Cas systems can be broadly classified into two classes: Class 1 systems are composed of multiple effector proteins that together form a complex around a crRNA, and Class 2 systems that consist of a single effector protein that complexes with the crRNA to target DNA or RNA substrates. The single-subunit effector compositions of the Class 2 systems provide a simpler component set for engineering and application translation, and has thus far been important sources of programmable effectors. The discovery, engineering, and optimization of novel Class 2 systems may lead to widespread and powerful programmable technologies for genome engineering and beyond.